Rabies is an ancient disease. It is well described in writings by Egyptians dating back to 2300 B.C. Rabies disease is caused by a virus that is present predominately in the saliva of rabid animals. The virus is transmitted by the bite of an infected animal. Rabies virus causes an inflammation of the brain and is almost always fatal once symptoms develop. In wild and domestic animals, the rabies virus may affect the part of the brain which regulates aggression, causing the animal to attack without fear or provocation. The rabies virus may also cause other changes in animal behavior. Wild animals that are normally out only at night may be seen during the day, approaching humans and domestic pets that they ordinarily would avoid.